Alzheimer's disease (AD) is the most common cause of progressive cognitive deficits in elderly people. This as yet incurable disease threatens the health of an increasing number of the elderly in the United States. The financial costs of caring for these patients are enormous. Evidence suggests that APP plays an important role in AD pathogenesis. However, basic functions of APP in the CNS remain largely obscure. Our long term goals are to characterize the functions of APP in the CNS and to determine if age-related dysfunction of APP contributes to neurodegeneration in AD. In our preliminary studies, we found that wild-type, but not familial AD (FAD)-linked APP, protected against apoptosis in an APP-deficient neuroblastoma cell line (B103). APP's antiapoptotic function correlated with its ability to inhibit a key pro-apoptotic molecule, tumor suppress factor p53. In the proposed studies, we will dissect mechanisms underlying APP-mediated inhibition of p53 and apoptosis. First, we will determine if APP decreases p53 activity by inhibiting p53 C-terminal phosphorylation, which regulates p53 activation. Neuronal cell cultures will be challenged with different apoptosis inducers, and the effect of APP on p53 C-terminal phosphorylation will be measured by western blot analysis with antibodies specific to the phosphorylated C-terminus of p53. The APP's ability to inhibit C-terminal phosphorylation and activation of p53 will be analyzed in B103 cells expressing wild-type, FAD-mutant, or frameshift-mutant APP (a truncated APP found in sporadic AD). Next, we will determine the APP-mediated signaling events responsible for p53 inhibition. APP may function as a cell-surface receptor relaying outside signals into cells, which may be responsible for p53 inhibition. To test this hypothesis, we will generate a chimeric APP receptor and determine if chemically-induced dimerizaiton of the chimeric APP receptor will inhibit p53 activation and apoptosis in neuronal cells. If so, we will then determine if the intracellular region of the APP receptor and recently characterized proteins that interact with APP, such as Fe65 and X11, are required for APP's p53 inhibitory function. We will also determine if FAD-linked mutations diminish APP's receptor- mediated function in neuronal cultures. Dissecting the molecular mechanisms underlying APP's regulation of neuronal cell death and survival will greatly expand our understanding of the role of this AD genetic marker in the development of AD.